Electrodeionization (EDI) is a membrane separation deionization technique that combines the techniques of electodialysis and ion exchange. EDI purification apparatus has many advantages, such as, producing water continuously, regenerating ion exchange resins without using alkalis and acids, automatically operating, etc. It has gradually replaced mixed bed as the final water treatment apparatus used in pure water preparation systems. After having been studied, practiced and developed for more than fifty years, EDI technique began to be recognized by more and more people and widely applied to medicine, electronic, electric power, and chemical industries, due to its environment friendly and easy to operate characteristics.
The earliest published and commercialized EDI apparatus all have a “stacked plate” structure, which is a mature structure made by a technique derived from electodialysis. However, the structure has disadvantages of poor pressure endurance, ease to leak during operation, high maintenance (the filled resins can't be replaced), easily producing leakage of electric field due to its open electric field which results in high energy consumption. CN 9822354.3 and U.S. Pat. No. 6,190,528 disclose a helical wound membrane electrodeionization apparatus, which overcomes the deficiencies aforementioned. The helical wound structure has a concentric and closed electric field with low energy consumption. In particular, the novel separated structure of the helical wound structure EDI enables the convenient replacements of the membrane module and resins, making it easier for maintenance.
In the prior art, the existence of organics in pretreated water often causes irreversible damages to the membranes of the electrodeionization apparatus, especially to filled ion exchange resins. The damages are more evident in the case of anion resins. Given gel resin as an example, the resin particulates as a whole is completely comprised of interior channels defined by interconnected cracks with different sizes. Generally, the cracks' size is between 20 and 40 Å, and the cracks' surfaces are distributed with active functional groups. The functional groups could release ions, which may further exchange with ambient external ions. When being adsorbed by the resins (usually anion resins), the organics in the water will attach to the surfaces of the resin particulates and their network cracks, causing difficulties for the diffusion of ions. This kind of adsorption is very stable and difficult to desorb, especially in the case that the organics enter the resin particulate structure and reach the interior exchange positions, where the organics will twist each other and can hardly be separated from the resin structure with any commonly used regeneration methods. As a result, the overall exchange capacity of resin is decreased. Moreover, the carboxyl group of the organics often changes the polarity of the exchange groups in this region.
In the dilute flow of the EDI module, the mobile ions absorption characteristic of the functional groups forms a “fast passage,” through which the inorganic ions in solution migrate to membrane surface under direct current. Compared with the volume of continuously flowing water feed through the electrodeionization apparatus, the amount of the ion exchange resins filled in EDI is very small. As a result, once the ion exchange passage is contaminated by the organics, the impact on the overall performance of the electrodeionization apparatus is much more severe than that on a common deionization (DI) unit. Since the contaminated resins are difficult to be regenerated, if the structure of EDI makes it impractical to replace the resins, the entire EDI unit will be discarded.
During operation of the EDI apparatus, in order to ensure that the filled resins are substantially and continuously regenerated, the total organic carbons (TOC) of the feed water must be maintained at an amount below 0.5 ppm. In practice, particularly in the case of treating surface water, shallow groundwater or polluted groundwater, although the feed water will be generally pretreated with a conventional reverse osmosis process and the TOC of the product water will be controlled to a level below 0.5 ppm, the EDI apparatus breakdown frequently due to the accumulation of pollutants on the filled resins over long time of operation. To solve the problem, manufactures applied two-stage reverse osmosis process, i.e. the water produced in the first reverse osmosis process will flow into another reverse osmosis module, to produce pretreated water with a TOC below 200 ppb. However, this leads to high energy consumption and high investment cost. Practically, in order to improve the performance of the EDI apparatus, it is effective and economic to replace the filled resins and even the membrane module when the filled resin has been accumulated with contaminant over time.
In the prior art, technicians in the field have attempted to solve many technical problems coming out during practical operation of the EDI apparatus, which include:
The ability of removing weak electrolytes such as silicon, boron etc is one of the most important factors for measuring the quality of water produced by EDI apparatus. CN1585727 discloses an EDI apparatus with improved removal ratio of silicon and boron in the feed water, wherein ion exchange resins are filled in the desalting chamber and the concentrating chamber; part of the product water is recycled to water inlet to reduce the concentration of; and the concentrated water is discharged. A water treatment system with a low boron content detection control device is disclosed in WO 03031034, wherein a desired desalting effect was obtained by detecting the boron content in the water treatment system and controlling the current and voltage of a EDI apparatus.
Scaling problem of EDI apparatus: U.S. Pat. No. 6,149,788 discloses a method and an apparatus for inhibiting the scale formation of the electrodeionization apparatus system, and particularly for improving the tolerance of the apparatus to the hardness of feed water by inhibiting the precipitation of metal ions in feed water, thereby improving the water treatment efficiency of apparatus.
CN 1615273 disclosed a fractional electrodeionization method for treating a liquid, wherein ions are sequentially removed in term of their ionic intensities without the formation of precipitation or scale. In the method, the whole EDI apparatus is divided into two sections, and the operation voltages for each of sections are adjusted according to the water quality, respectively.
The aforementioned CN 9822354.3 and U.S. Pat. No. 6,190,528 also disclose a helical wound EDI apparatus. In the apparatus, an insulated net-separating partition is disposed between a pair of anion and cation exchange membranes to form a special membrane bag. The opening side of the membrane bag is in liquid communication with water gathering aperture slotted in a side wall of the concentrated-water-gathering pipe to form a concentrated water chamber. An insulated net is placed between the adjacent membrane bags, and the two ends of the net are sealed by adhesives to form a dilute chamber into which ion exchange resin particulates can be filled. Then, they wound around the concentrated-water-gathering pipe to form a cylinder structure. The cylinder is then wrapped by a metal crust from outside, which is in turn coated by an insulated polymer layer. Meanwhile, a filter and a cover are fitted in both ends of the cylinder membrane module. The above-mentioned structure has many advantages, such as, simple in structure, highly effective availability of membrane, low resistance of dilute chamber, less pressure drops, no leakage etc. The structure overcomes the deficiency of the stacked mechanical sheet type EDI, solves the technique problems of scaling and weak electrolyte (e.g., silicon and boron) removing; particularly, it overcomes the difficulty in timely and conveniently replacing the filled resins once they are irreversibly contaminated during the operation of the EDI apparatus, thereby reducing the cost and requirement of pretreatment.
However, this helical wound structure has a defect of uneven distribution of current density. The electric field between the concentric designed cathode and anode is radiate. Therefore, the electric field becomes stronger when getting closer to the central pipe, and becomes weaker contrarily. As a result, desalting performance varies in each section of EDI apparatus, with the closer to the center, the better the performance.
U.S. Pat. No. 5,376,253 discloses a spiral wound EDI apparatus, wherein the cathode and anode are concentrically disposed, and the dilute water flows spirally and radially from the periphery side with weak electric field to the center with strong electric field along radial direction. This solves the problem of the current density difference between the inside and outside. However, in order to ensure the water yield of the apparatus, this structure has to be lengthened in axial direction, hence increasing the length of electrode. Furthermore, the resistance of water flow increases due to the spiral flow of the dilute water, and the structure is complex, making it difficult to fill and replace the resins.
CN 1426970A discloses a wound electodialysis apparatus of circulating concentrated and dilute water, wherein the dilute water flows into the apparatus and flow out through central pipe which is a U-shaped flow passage inside the membrane bag. The concentrate flows through the flow passage along axial direction, and directly flows through the module, and flows out through the outlet in the side of apparatus. This design partially solves the problems caused by the unequal intensity of electric field. However, the design of the flow passage of the concentrate and the dilute is undesirable, making the pressure of the concentrated water higher than that of the dilute in certain areas, which results in the reverse permeation of the concentrate into the dilute chamber, adversely affecting the quality of the product water.